1. Field of the Invention
This invention relates to an automatic transmission installed in an automotive vehicle.
2. Discussion of the Related Art
Automatic transmissions having a torque converter and a transmission gear mechanism to automatically change the speed stages are known in the art. The torque converter changes the torque speed of an engine output shaft and transmits the resultant torque to a turbine shaft. The transmission gear mechanism changes the torque speed of the turbine shaft, and transmits the resultant torque to drive wheels of a vehicle.
The transmission gear mechanism includes a planetary gear comprising a sun gear, a ring gear, pinion gears, a carrier, and the like. A variety of friction elements are provided in the transmission gear mechanism, such as a clutch which can be engaged to transmit torque to a specified gear or carrier, and a brake for locking or releasing the specified gear or carrier. An engagement pattern of these friction elements is changed using, for example, a hydraulic mechanism. Thus, the gear is shifted by switching the speed stages step-wise.
In an automatic transmission provided with the above transmission gear mechanism, more speed stages in the transmission gear mechanism result in a greater selection of torque transmission characteristics. This allows a vehicle to be driven in a manner suitable to road or running conditions, and thereby improves vehicle mileage and performance. However, with a single transmission gear mechanism only a limited number of speed stages can be provided, which is normally a maximum of four forward gears.
In view of the above, there has been proposed a multistage automatic transmission in which a main transmission and sub-transmission are provided in series (with respect to a direction of torque transmission). Combining the speed stages of the two transmissions allows the automatic transmission to have an increased number of speed stages. The above automatic transmission is disclosed, for example, in Japanese Unexamined Patent Publication No. 62-4950.
By combining the speed stages of the main transmission and the sub-transmission, an automatic transmission may have six forward gears, for example, when a main transmission having three forward gears and a sub-transmission having two forward gears are connected in series. An automatic transmission having five forward gears may be obtained by deleting any one gear from this automatic transmission having six forward gears.
When the main transmission and the sub-transmission are combined to construct one multistage automatic transmission, a main transmission is mounted on a first shaft (turbine shaft), which is connected to an engine output shaft through a torque converter, and a sub-transmission for changing and transmitting an output of the main transmission through a second shaft (main shaft of sub-transmission). It is known that the two transmissions are arranged so that axes of the respective shafts are spaced apart by a specified distance in parallel with each other. In this case, the transmission mechanisms of the two transmissions are coupled through a torque transmission means such as gears. The spacing between the axes of the first shaft and the second shaft is determined according to a position where the torque transmission means is permitted to operate in cooperation with the two transmission mechanisms and dimensions thereof.
The output of the sub-transmission is normally input directly to a differential of the vehicle, and the sub-transmission is normally arranged above the differential. Accordingly, the main transmission and the sub-transmission are generally arranged so that the level of the axis of the second shaft is at or above the level of the first shaft.
The main transmission and the sub-transmission each require a hydraulic housing to control supply and discharge of hydraulic pressure to and from the friction elements provided in the respective transmissions. When these valve bodies are mounted in an automatic transmission, the hydraulic housing for the main transmission, which is designed to set a line pressure of the hydraulic pressure to be supplied, is incorporated in an oil pan of a gear shifter arranged below the main transmission.
However, it is difficult to incorporate both the hydraulic housing of the sub-transmission in the oil pan and the hydraulic housing of the main transmission because of spatial limitations. In this case, it is important to lay out an oil path for connecting the hydraulic housing with the transmission mechanism of the sub-transmission which is as short as possible in order to ensure sufficient hydraulic responsiveness and improve controllability in the sub-transmission.
However, the hydraulic housing needs to have accessories such as a solenoid valve for switching hydraulic pressure and an oil path connecting portion which connects the oil path with the sub-transmission mounted in the same vicinity. Thus, it has been considerably difficult to design an hydraulic housing for controlling the hydraulic pressure which contains all the necessary accessories and oil paths in the small area around the gear shifter without problems and with high efficiency.
Accordingly, it is an object of the present invention to provide an automatic transmission which has overcome the above mentioned problems.
It is another object of the present invention to provide a multistage automatic transmission which is capable of shortening an interconnecting oil path with a transmission mechanism of a sub-transmission as much as possible and laying out accessories and a connecting portion of the interconnecting oil path without problems.